The present invention relates, in general, to the field of thin film devices and processing. More particularly, the present invention relates to a method for forming a re-entrant photoresist lift-off profile for thin film device processing of particular utility in conjunction with self-aligned sputtered films, such as permanent magnet ("PM") films, for use in magnetoresistive ("MR") read heads.
In the processing of thin film devices, such as magnetoresistive heads, a "lift-off" process may be used to selectively remove thin, sputtered films from patterned areas on the layers overlying the device substrate. In this regard, re-entrant (or undercut) photoresist sidewalls are used to define the patterned area. Utilizing re-entrant sidewalls, the upper portion of the photoresist feature is wider than its base in order to facilitate subsequent removal of the photoresist by a suitable solvent.
One common approach to achieving a re-entrant photoresist sidewall as been through the use of an image reversal photoresist process in which a positive tone photoresist is rendered negative by means of post exposure bake and flood expose steps. In this manner, an undercut may be generated which is suitable for some applications. Still further, double photoresist layers may be used to define the patterned areas wherein two separate overlying layers of photoresist may be patterned, with the lower layer being more soluble than the upper layer. Because of the differing etch rates, exposure to a standard chemical bath can generate a desired undercut also suitable for limited applications.
Thin film permanent magnet layers are often utilized with MR read heads, or sensors, to provide a longitudinal bias to the active MR region in order to provide Barkhausen noise suppression. Desirably, the permanent magnet layers are applied through the use of a self-aligned process in order to ensure their proper and uniform alignment with the MR element. This self-alignment is achieved by depositing a thin permanent magnet film (in conjunction with a non-magnetic metal or dielectric separation layer) onto a previously defined MR element that is protected by photoresist material. The MR element may comprise, for example, a multi-layer magnetoresistive structure ("MRS") on a suitable substrate, wherein the MRS includes an MR layer overlying a magnetic spacing layer ("MSL") in conjunction with an underlying soft adjacent layer ("SAL") to provide transverse bias to the MR element. The active region of the MR element may be defined by use of photoresist appropriately patterned on the MRS to allow, for example, etching away portions of the MR layer and underlying MSL and SAL layers. Leaving the photoresist over the active MR region and then depositing the separation and PM layers places the permanent magnet layers in correct alignment with the MR active region. Subsequent removal of the photoresist by etching in a suitable solvent removes the PM material overlying the photoresist as well while leaving the desired PM material properly positioned adjacent the MR element. The foregoing United States patent discloses a process for effectuating the foregoing.
The re-entrant, or undercut, photoresist profile described above is, in this application, extremely useful to enable a suitable solvent to penetrate the photoresist material to etch it away. This is because if the overlying PM material conforms to the entire resist profile, the solvent will be precluded from reaching the photoresist and dissolving it. Moreover, it has heretofore been extremely difficult to consistently pattern undercut profiles in photoresist using standard image reversal and double photoresist layer processes, particularly with device features in the range of 1.5 .mu.m or less. It is therefore highly desirable to provide an improved method for forming a re-entrant photoresist lift-off profile for thin film device processing which is of particular utility in conjunction with self-aligned sputtered films, such as permanent magnet films, for use in MR read heads.